This invention relates to methods and apparatus for measuring variations in distance to a surface, particularly for determining surface profiles or contours, and can also be used for measuring surface displacements.
The present invention is of particular value for profile measurements where the surface roughness is comparable with, or larger than, the wavelength of visible light, as with some machined surfaces. With specularly reflecting surfaces such measurements can readily be made using a conventional Michelson interferometer, but with such non-specular surfaces it is difficult or impossible to count the fringes as the surface moves laterally relative to the light beam of the interferometer; fringes due to displacement variations can be counted, but not profile variations.
The use of projected interference fringes generated by two beams of plane waves to contour an object surface has been described by Rowe and Welford in Nature, Vol 216, p786 and Optica Acta, Vol 16, p371. An array of sheets of light thus generated is caused to illuminate an object volume and a bright fringe appears wherever the object surface intersects one of these sheets. The fringes can thus be used to build up a contour map of the object.
The present invention provides a method which employs the above principle but which permits increased sensitivity and allows automatic read-out of the measured contour, and which can be used for rough surfaces.